The present invention relates to a dual band feedhorn and orthomode transducer (OMT) for use with a terrestrial satellite parabolic reflector.
Ideally, a dual band feedhorn should be capable of simultaneously illuminating an offset parabolic reflector (with an F/D ratio of about 0.5) at two frequencies, e.g. the Ku and Ka band. The antenna beams produced at both bands should be centred along the same boresight axis. This requires the use of one single feed for both bands.
The main function of the OMT is to provide isolation between the signals at two frequencies, for example the Ka and Ku bands. The OMT should be capable, for instance, of simultaneously transmitting both polarisation directions (vertical and horizontal) of the Ku band from the feedhorn to the Ku band port, and be capable of transmitting one of both polarisation directions (vertical or horizontal) of the Ka band from the Ka band port to the feedhorn. This means there are two possible versions of the OMT depending on the Ka band polarisation direction.
U.S. Pat. No. 5,003,321 describes a dual frequency feed which includes a high frequency probe concentrically mounted with a low frequency feed horn. A concentric circular waveguide has a first turnstile junction mounted adjacent the throat of the low frequency feed, which branches into four substantially rectangular, off axis waveguides extending parallel to the central axis of the waveguide. These waveguides and the low frequency signals conducted through them are then recombined in a second turnstile junction which is coaxial with the low frequency feed, high frequency probe and first turnstile junction. The high frequency feed is introduced in between two of the four parallel off-axis waveguides. The known device is split longitudinally. This split results in complex joining and sealing surfaces at the end of the low frequency feed horn and at the position where the high frequency probe is lead off axis.
The present invention may provide a dual band, higher and lower frequency range transducer with a circular coaxial waveguide feed, a first junction for connection of a lower frequency range outer waveguide of the coaxial waveguide feed to at least two rectangular or ridge waveguides offset from the longitudinal axis of the transducer, a second junction for connection of the at least two rectangular or ridge waveguides to a further waveguide and a third junction for connecting an inner waveguide of the coaxial waveguide feed to a higher frequency range waveguide, characterised in that the transducer is formed from at least two parts joined across a first plane perpendicular to the longitudinal axis and including a part of the higher frequency range waveguide within the join. By xe2x80x9chigher and lowerxe2x80x9d frequency is meant that there is a frequency difference between the higher and lower ranges. Typically, there is no overlap between the ranges.
Preferably, a water seal is provided in the plane of the first join. Preferably, all of the junctions include impedance matching devices. A feed horn may be attached to the coaxial feed. The feed horn preferably has corrugations. The first and second junctions may be provided by further parts which are joined to the other parts along planes parallel to the first plane. The horn is preferably sealingly attached to the first junction part along a plane parallel to the first plane. Preferably, a dielectric rod antenna is located in the inner waveguide at the end facing the horn. The end of the inner waveguide is preferably provided with a device for preventing backscattering from the rod antenna. The device is preferably a flare opening outwards towards the horn.
The transducer of the present invention allows the attachment of a higher frequency waveguide to the inner waveguide of the coaxial waveguide such that the higher frequency waveguide extends at an angle to the longitudinal axis of the transducer. The higher frequency waveguide extends at substantially 90xc2x0 to the longitudinal axis of the waveguide. This distinguishes the present invention over those dual band transducers which extract both higher and lower frequency range waveguides parallel to the longitudinal direction.
The present invention will now be described with reference to the following drawings.